Radio signals were used to set off Dallas’ outdoor warning sirens late Friday, city officials said.

All of the city’s 156 outdoor sirens that typically warn people when there is severe weather were activated around 11:40 p.m. Friday. They went off sporadically for about an hour and a half. Emergency crews had to manually turn each siren off early Saturday morning, and that lead to a surge in 911 calls.

City Manager T.C. Broadnax said the FBI is assisting the Dallas Police Department to find the person responsible. Investigators believe the hack using the radio signals came from the Dallas area.

UT-Dallas data security expert Dr. Murat Kantarcioglu says it appears Dallas did not have basic security safeguards on the system.

“Whenever they have these systems, they need to do cyber risk assessments, not only for this, but other systems the city is operating,” Dr. Kantarcioglu said.

City Manager T.C. Broadnax said city officials are also examining the city's water, 911, dispatch and financial systems for vulnerabilities after the hack.

“We will work to identify and prosecute those responsible,” Dallas Mayor Mike Rawlings said. “This is yet another serious example of the need for us to upgrade and better safeguard our city's technology infrastructure. It's a costly proposition, which is why every dollar of taxpayer money must be spent with critical needs such as this in mind. Making the necessary improvements is imperative for the safety of our citizens.”

The city’s Office of Emergency Management said more safeguards have been put in place to prevent such a takeover from happening again.

This hack also put added pressure on the city's already strained 911 call center. Technology issues combined with low staffing levels has prevented some people from getting through to 911 in recent months. The average hold time was 6 minutes during the siren snafu.